Is it possible to add fonts?

I'll take that as Marketo haven't supported the feature yet. I'm going to post it to the "Idea" section.

Yes please - our corporate site uses Calibri throughout so I cannot maintain our style guide in Marketo campaigns...

That is unrelated to Marketo and/or Design Studio.

Calibri  is a sans-serif typeface family under the Microsoft ClearType Font Collection. In Microsoft Office 2007, it replaced Times New Romans the default typeface in Word[1] and replaced Arial as the default in PowerPoint, Excel, Outlook, and WordPad.

Systems not running Micorsoft Office will select a fallback option. 

I am expecting some users argue most corporate environments run MS-Office. While that is still relatively the norm it is important to highlight mobile devices are outselling PC for years in a row. They do not offer native Calibri font. 

As a hardliner Apple and Unix/Linux user, I avoid any non-standard content of styles. Just for the record and perhaps laugh, I often read emails in text-mode only (mutt in all Unix-like, including Mac OS X). It is a lot faster and simpler to read than distracting HTML emails. Text-only browsing is also much faster and focused.

Web safe fonts: http://www.w3schools.com/cssref/css_websafe_fonts.asp

Marketo can't support Chinese Font as well,